New command /layout has been added and a new option weechat.look.save_layout_on_exit. WeeChat now saves layout (windows layout and order of buffers). With /layout command, you can show saved layout, apply layout, and save layout for windows and/or buffers.

Windows layout is position of windows (including size), and buffer displayed in windows (plugin and buffer name).
Buffers layout is order of buffers, ie numbers assigned to buffers.